De novo fatty acid synthesis

The general mechanism of fatty acid biosynthesis in plants is well recognized (Harwood, 1988, 1989 Stumpf, 1989 Hills and Murphy, 1991 Somerville and Browse, 1991). De novo synthesis involves the conversion of acetyl-CoA to Cie and Cig fatty acyl moieties in seed and mesocarp 

2J AcetyUCoA carboxylase. Incorporation of metabolically derived acetyl-CoA into fatty acid synthesis is believed generally to involve three major enzyme systems, namely acetyl-CoA carboxylase (EC 6.4.1.2), [acyl-carrier protein] (ACP) acetyltransferase (EC 2.3.1.38) 

The apparent ready reversibility of the reactions catalyzed by [ACP] acetyltransferase and [ACP] malonyltransferase is not suggestive that these enzymes are appropriate candidates for consideration as major regulatory enzyme systems (Walsh et al., 1990 Post-Beittenmiller et al., 1991). This conclusion is supported by the finding that [ACP] acetyltransferase showed very low specific activity in developing seeds of Cuphea lutea, rape and safflower (Shimakata and Stumpf, 1983). None the less under in vitro conditions, increased levels of [ACP] acetyltransferase enhances the synthesis of medium-chain fatty acids (Stumpf, 1989). This observation may only serve to emphasize the definite involvement of this enzyme system in fatty acid synthesis without implying that it is under major regulation. 

2 Fatty acid synthase. Fatty acid biosynthesis in plants occurs primarily in the plastids and is dependent upon the existence of the fatty acid synthase (or synthetase) II complex (FAS) (Harwood, 1988 Stumpf, 1989). This complex, unlike the system present in animals, is discontinuous (Shimakata and Stumpf, 1982a). Thus it has been possible to isolate and study the associated enzyme activities independently. Purification of each activity to homogeneity can then allow amino acid sequence analysis, cloning and elucidation of the gene, and subsequent application in the genetic engineering of plants. 

Acyl-carrier protein. Mechanistically FAS activity is critically dependent upon acyl-carrier protein (ACP) as cofactor for the several associated enzyme activities (Guerra and Holbrook, 1988 Ohlrogge, 1987, 1988 Ohlrogge et al., 1991). A particular relevance can be attached to the observation that increased levels of ACP accompany storage lipid synthesis in soybean seed (Ohlrogge and Kuo, 1984). Due to the close 

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In the chloride shift, Ck plays an important role in the transport of carbon dioxide (qv). In the plasma, CO2 is present as HCO, produced in the erythrocytes from CO2. The diffusion of HCO requires the counterdiffusion of another anion to maintain electrical neutraUty. This function is performed by Ck which readily diffuses into and out of the erythrocytes (see Fig. 5). The carbonic anhydrase-mediated Ck—HCO exchange is also important for cellular de novo fatty acid synthesis and myelination in the brain (62). 

There are also microorganisms that can produce poly(HAMCL)s when grown with substrates that are much different from those discussed above, such as glucose [42-44]. The 3HA monomers produced by these microorganisms were most likely obtained from intermediates of the de novo fatty acid synthesis route [45]. PHAs synthesized from unrelated organic substrates are described in Sect. 3.1.2. 

There is considerable interest in synthesizing copolymers. This is actually possible if organisms are confronted with mixtures of so-called related and unrelated substrates. Copolymers can also be synthesized from unrelated substrates, e.g., from glucose and gluconate. The 3-hydroxydecanoate involved in the polyester is formed by diversion of intermediates from de novo fatty-acid synthesis [41,42]. Related , in this context, refers to substrates for which the monomer in the polymer is always of equal carbon chain length to that of the substrate offered. Starting from related substrates, the synthesis pathway is closely connected to the fatty-acid /1-oxidation cycle [43]. In Pseudomonas oleovor-ans, for example, cultivated on octane, octanol, or octanoic acid, the synthesized medium chain length polyester consists of a major fraction of 3-hydroxyoc-tanoic acid and a minor fraction of 3-hydroxyhexanoic acid. If P. oleovorans is cultivated on nonane, nonanol, or nonanoic acid, the accumulated polyester comprises mainly of 3-hydroxynonanoate [44]. 

The fatty acids of bovine milk fat arise from two sources synthesis de novo in the mammary glands and the plasma lipids originating from the feed. The fatty acids from these two sources differ in their structure. The fatty acids that are synthesised de novo are short-chain and medium-chain length acids, from 4 0 to 14 0 and also some 16 0, while the Cis fatty acids and some 16 0 arise from the plasma lipids. De novo fatty acid synthesis accounts for approximately 45% (w/w) of the total fatty acids in milk fat, while lipids of dietary origin account for the rest (Moore and Christie, 1979). 

Loor, J.J., Herbein, J.H. 1998. Exogenous conjugated linoleic acid isomers reduce bovine milk fat concentration and yield by inhibiting de novo fatty acid synthesis. J. Nutr. 128, 2411-2419. 

Figure 1 Fatty acid synthesis in mammals. Gene encoding enzymes are shown in italics and are based on current evidence for substrate specificities. (A) De novo fatty acid synthesis. (B) Synthesis of the highly unsaturated fatty acids AA, EPA, and DHA from Cl 8 2 oo3 and Cl 8 3 m3 obtained from the diet. Details are found within the text.

In addition to its effect on insulin secretion, GLP-1(7 36) amide stimulates de novo fatty acid synthesis in adipose tissue (Oben etal., 1991). GLP-1(7 36) activates adenylate cyclase (Drucker et al., 1987 Conlon, 1988) this peptide probably stimulates insulin release in a similar manner to glucagon. 

The first step in de novo fatty acid synthesis is the production of malonyl-CoA from acetyl-CoA and bicarbonate. This committed step is catalyzed by acetyl-CoA carboxylase present in the cytoplasm of liver cells and adipocytes. After replacement of the CoA residue in acetyl-CoA by ACP (acyl carrier protein), malonyl-ACP is used to convert acetyl-ACP to butyryl-ACP by the fatty acid synthase complex. In this multistep reaction, NADPH is used as donor of hydrogen atoms and CO2 is produced. Butyryl-ACP is subsequently elongated to hexanoyl-ACP by a similar process in which malonyl-ACP serves as donor of two carbon atoms required for lengthening of the growing acyl chain. This process is repeated until palmitic acid... 

Note that the primary use for glucose in fat cells is as the precursor or glycerol. Fat cells do not carry out significant de novo fatty acid synthesis. 

The data show there is little consensus on the phase that tumor cells arrest growth after inhibition of FASN in various tumor cells, which may be attributed to different tumor cell types. It is likely that a lack of de novo fatty acid synthesis in tumor cells impacts phospholipid synthesis required for proper DNA synthesis and cell cycle progression (Jackowski, 1994). 

The existence of a mitochondrial pathway for de novo fatty acid synthesis was first reported 50 years ago, when it was generally assumed that fatty acid synthesis proceeded by reversal of the mitochondrial pathway for fatty acid P-oxidation (F. Lynen, 1957). However, the discovery of the cytosolic malonyl-CoA pathway (R.O. Brady, 1958 S.J Wakil, 1958) casted doubt on these claims and interest in this system waned until the discovery that mitochondria of both Neurospora crassa and Saccharomyces cerevisiae contain nuclear-encoded mitochondrial proteins that function as a type II FAS system. Disruption of the genes encoding these enzymes in both N. crassa and S. cerevisiae produces respiratory-deficient phenotypes and in S. cerevisiae cellular lipoic acid is reduced to less than 10% of that of the wild-type strain (R. Schneider, 1995 E. Schweizer, 1997). These observations suggested that in fungi one of the roles of this pathway might be to generate the lipoyl moieties required for mitochondrial function. 

De novo fatty acid synthesis is, in part, negatively regulated by the AMP-activated protein kinase (AMPK). When the adipocyte is in an energy deficient state due to the lack of available glucose or other cellular stresses, ATP levels decrease and AMP levels increase. This causes the AMPrATP ratio in the adipocyte to increase and thus AMP binds and activates AMPK kinase, LKB. AMP also binds to AMPK, which allows LKB to recognize AMPK as a substrate and results in the phosphorylation and activation of AMPK. 

Sethoxydim (e.g., Poast ) inhibits acetyl CoA carboxylase (ACCase) which is the first committed step in de novo fatty acid synthesis. SR corn was achieved by traditional breeding and selection for the herbicide insensitive ACCase allele and was introduced in 1996. SR corn accounted for less than 0.3% of corn acres in any one year and is no longer commercially available in field com (Fig. 6.1.4). 

Yang, L.-Y., Kuksis, A., Myher, J. J. and Steiner, G. (1996) Contribution of de novo fatty acid synthesis to very low density lipoprotein triacylglycerols evidence from mass isotopomer distribution analysis of fatty acids synthesised from pH6] ethanol. J. Lipid Res., 37, 262-74. 

The most important saturated fatty acid in higher plants is palmitic acid. Stearic acid, in contrast, occurs in low amounts as an acyl component of complex lipids. While the individual enzymes for the synthesis of palmitic acid have not been examined, there is no evidence at present that they are associated. There is now increasing evidence that the initial product of de novo fatty acid synthesis in plants is palmitoyl-ACP. The components of systems for the synthesis of palmitoyl-ACP include ACP, NADPH, NADH, acetyl-CoA, and malonyl CoA. In support of this, the following observations may be cited ... 

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